The present disclosure relates to refrigeration systems. More specifically, the present disclosure relates to refrigeration systems with multiple heat transfer fluid circulation loops.
Refrigerant systems are known in the HVAC&R (heating, ventilation, air conditioning and refrigeration) art, and operate to compress and circulate a heat transfer fluid throughout a closed-loop heat transfer fluid circuit connecting a plurality of components, to transfer heat away from a secondary fluid to be delivered to a climate-controlled space. In a basic refrigerant system, heat transfer fluid is compressed in a compressor from a lower to a higher pressure and delivered to a downstream heat rejection heat exchanger, commonly referred to as a condenser for applications where the fluid is sub-critical and the heat rejection heat exchanger also serves to condense heat transfer fluid from a gas state to a liquid state. From the heat rejection heat exchanger, where heat is typically transferred from the heat transfer fluid to ambient environment, high-pressure heat transfer fluid flows to an expansion device where it is expanded to a lower pressure and temperature and then is routed to an evaporator, where heat transfer fluid cools a secondary heat transfer fluid to be delivered to the conditioned environment. From the evaporator, heat transfer fluid is returned to the compressor. One common example of refrigerant systems is an air conditioning system, which operates to condition (cool and often dehumidify) air to be delivered into a climate-controlled zone or space. Other examples may include refrigeration systems for various applications requiring refrigerated environments.
However, many proposed systems having two-phase CO2 as a secondary heat transfer fluid require the CO2 to be maintained in a supercritical fluid state, which can add to equipment and operating complexity and cost. Further, conventional operation, especially startup, of such a system can result in operational inefficiency and pump cavitation in the secondary heat transfer loop.